Importance of Sheet Resistance Mapping of Semiconductor Wafers
The invention described in this specification may be applied generally in computer controlled machines which perform various production or testing processes. It may also be applied to data collection and data base management programs. However, the detailed description of the invention will be given in terms of the control of an automated resistivity tester for performing sheet resistance mapping of semiconductor wafers. This equipment is used to characterize the performance of semiconductor wafer manufacturing equipment utilized to form surface layers of specific target conductivity value as part of the process of manufacturing semiconductor devices such as, for example, large scale integrated circuits.
The preferred vision of an automated resistivity tester to be controlled by this invention is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 726,498, filed on Apr. 24, 1985, and entitled "APPARATUS AND METHODS FOR SEMICONDUCTOR WAFER TESTING." This disclosure is specifically incorporated herein by reference. The use of computer controlled testing apparatus of this type as the background environment for demonstrating the advantages of this invention is especially meaningful because of the importance of the semiconductor industry to the advancement of science and technology in many areas, including areas of factory automation to which this invention may be very meaningfully applied. To understand the overall importance of automated resistivity testing to the semiconductor industry, reference is made to the helpful background information given in the above-identified copending application on the status of the industry and the particular importance of performing automated resistivity testing on semiconductor wafers which have been subjected to ion implantation.
The correctness and uniformity of implant dosage across a semiconductor wafer can be determined in an automatic sheet resistance mapping system which has the capability of taking multiple test readings in both a contour map and diameter scan mode. It is well-known in the prior art to utilize an in-line four-point probe to make sheet resistance measurements at various preselected testing locations on the implanted semiconductor test wafer. Sheet resistance mapping and the theory and results of such testing techniques are discussed in detail in the following papers and publications, which are incorporated herein by reference:
D. S. Perloff et al., "Dose Accuracy and Doping Uniformity of Ion Implantation Equipment," Solid State Technology, February 1981, pp. 112-120;
Andrew B. Wittkower, "The Effect of Ion Implanter Design Upon Implant Uniformity," Solid State Technology, September 1982, pp. 77-81;
David S. Perloff, "Four-Point Probe Correction Factors for Use in Measuring Large Diameter Doped Semiconductor Wafers," Journal of the Electrochemical Society: Solid-State Science and Technology, November 1976, pp. 1745-1750; and
D. S. Perloff, et al., "Contour Maps Reveal Non-Uniformity in Semiconductor Processing," Solid State Technology, February 1977.
FIG. 1 illustrates a printout of a contour map which shows a pattern of non-uniform implantation of a wafer. The heavy contour line represents the mean resistivity value of the wafer. The lighter contour lines outline regions which differ from the mean value by a preset percentage. The + and - symbols represent test sites for which the resistivity value is above and below the mean value, respectively. FIG. 2 illustrates a printout of two diameter scans on the same wafer. Here the actual resistivity values are plotted as a function of position of the test site on the diameter of a wafer. From these tests and printouts, the engineer in charge of a process can determine whether the ion implantation equipment is operating properly. Specific discussion of the diagnostic capability of the resistivity tests is contained in the above-referenced articles.
In a co-pending and commonly assigned patent application entitled "Apparatus and Method for Resistivity Testing," Ser. No. 704,296, filed Feb. 22, 1985, a novel arrangement for orienting the resistivity test probe for improved accuracy in performing four-point probe sheet resistance measurements on conductive surface layers of a semiconductor wafer is disclosed. The specification of that application is hereby incorporated by specific reference.
To encourage the use of testing equipment such as automated resistivity testers, it is important to provide an overall computer control program for the tester which is easy for the engineer to set up to perform in-process monitoring measurements which will provide meaningful data. It is also important for the control program to be simple for the operator to run with confidence and consistency to produce meaningful data. Engineer and operator convenience and confidence are the keys to increasing acceptance of automated process control and testing in all industries.